Many printed circuit board are finished by electroless nickel/gold plating. This step follows etching of copper circuit and coating of solder resist, and deposits electrolessly plated nickel and gold films, selectively on exposed copper portions.
Electroless nickel plating follows several pretreatment steps, in which boards are immersed in a series of pretreatment solutions. One of the pretreatment steps is activator, in which Pd catalyst is attached on the exposed copper portions in order to initiate deposition of electroless nickel plating, which is followed by electroless gold plating.
Such an electroless nickel plating step, however, is frequently accompanied by a phenomenon in which nickel is deposited not only on the copper circuits but also on nearby insulator sections.
This phenomenon is called "bridging", "excess plating" or "bleeding", depending on the communities and the organizations.
Recently, printed circuit boards are increasingly required to have circuits with narrower width of line and space. This affects that probability of defects caused by bridging, such as short-circuit, highly increases.
Probability of occurrence of the bridging phenomenon significantly increase in boards prepared by build-up methods, which involve attachment of Pd catalyst on resin surface, electroless copper plating, electrolytic copper plating and formation of circuits by etching of the deposited copper layer. Pd catalyst is basically dissolved in copper chloride and iron chloride solutions used as etchant for circuit patterning. But, if it partially remains, bridging of electrolessly plated nickel are easily occurred on the residual Pd catalyst. As a result, it causes serious problem, because even one short-circuit can damage whole pattern.
Prevention of this phenomenon needs very strict control of the electroless nickel plating solution composition and stirring condition.
It is an object of the present invention to establish a technique which prevents "bridging" by electroless nickel/gold plating between the copper patterns.
The inventors of the present invention have found, after extensive studies to solve the above problems, that the "bridging" phenomenon can be efficiently prevented by immersing the board in a solution containing a thiosulfate immediately before the pretreatment for electroless nickel plating, comprising a series of steps of degreasing, soft etching, acid treatment and activator treatment.
Furthermore, they have also found that use of permanganate treatment and neutralization steps before the thiosulfate treatment is effective, when the Pd catalyst is deposited on the side walls of the holes, prepared in the boards by drilling or the other methods.